first make sure vboxdrv for current version of kernel is gone: (else modprobe will load the wrong one and thus fail)

sudo pacman -Rs linux315-virtualbox-host-modules
so now this one is gone from here:
/usr/lib/modules/extramodules-3.15-MANJARO/vboxdrv.ko.gz
but this one can still remain:
/usr/lib/modules/extramodules-3.12-MANJARO/vboxdrv.ko.gz
since I'm using stock 3.12 kernel

then now:
install this:
sudo pacman -S virtualbox-host-dkms
to be able to compile vboxdrv.

run this as instructed:
sudo dkms install vboxhost/4.3.12
to compile vboxdrv and related ones.

though they(in the virtualbox UI) say I should do this:
sudo dkms autoinstall
maybe do that also! when I tried it, seemed to have done nothing, maybe all checks passed:)


enable this to rebuild it whenever needed(I think! i think it's like for the fglrx driver):
sudo systemctl enable dkms.service

the module is build and put in:
/var/lib/dkms/vboxhost/4.3.12/3.15.3-1-MANJARO/x86_64/module/vboxdrv.ko
/usr/lib/modules/extramodules-3.15-MANJARO/vboxdrv.ko.gz

load it before starting up a virtualbox machine:
sudo modprobe -v vboxdrv

